1. Field of the Invention
The present invention relates to an arrayed waveguide grating (AWG: Arrayed Waveguide Grating) type optical multiplexer/demultiplexer applicable as a wavelength selection element to wavelength division multiplexing (WDM: Wavelength Division Multiplexing) transmission systems.
2. Related Background Art
The AWG type optical multiplexer/demultiplexers (hereinafter referred to as AWG circuits) are widely applied to the wavelength selection elements in the WDM transmission systems, as wavelength filters enabling extraction or insertion of a specific wavelength by interference. The AWG circuits do not require so precise machining as required by diffraction gratings or so precise multilayer film formation as required by interference films, but they can be constructed by ordinary microprocessing such as lithography, etching, and soon. Therefore, the AWG circuits are expected to develop into dominant optical components in the future WDM transmission systems, also including the possibility of integration with other light waveguide elements.
Such AWG circuits have the structure in which an input waveguide, an input slab waveguide, channel waveguides (phased array) of mutually different lengths, an output slab waveguide, and output waveguides are integrated on a single substrate.
FIG. 6 is a plan view to show the waveguide structure of light output part in a conventional AWG circuit (optical multiplexer/demultiplexer). In this AWG circuit, ends of channel waveguides 10 of mutually different lengths are connected at intervals d to one connection surface of output slab waveguide 20. Ends of output waveguides 30 provided corresponding to beams of respective channel wavelengths, i.e., corresponding to respective signal channels are connected to the other connection surface of the output slab waveguide 20.
In general, the connection surface of the output slab waveguide 20 to which the ends of channel waveguides are connected, is designed to work as a convex lens, and one end of each channel waveguide is placed on the circumference of a circle of the radius s (slab length) with the center at a position O1 where the light of the center channel wavelength converges. On the other hand, one end of each output waveguide connected to the other connection surface of the output slab waveguide is placed on the circumference of a Rowland circle 200 having the diameter equal to the slab length (reference is made to Japanese Patent No. 2599786 and xe2x80x9cApplied Optics I,xe2x80x9d the first edition published Jul. 20, 1990 by Baihukan).
However, the signal of the center channel wavelength was surely converged at the point O1, whereas there was the possibility that there occurred decrease of light collection efficiency and distortion of wavelength characteristics due to aberration or the like with the signals of the other channel wavelengths to be converged at points except for the point O1 on the Rowland circle. Specifically, there occurs loss variation of about 4 dB among the channel wavelengths, as illustrated in FIG. 7, in loss spectra of the respective output waveguides (hereinafter referred to as output CHs) in the conventional AWG circuit.
FIG. 8 schematically shows the loss spectrum (of the spectral width W1) in the output waveguide (output CH located near the center) corresponding to the center channel wavelength and the loss spectra (of the spectral width W2 ( greater than W1)) in the output waveguides (output CHs located near the periphery) corresponding to the longest channel wavelength and to the shortest channel wavelength in the signal wavelength band out of the loss spectra illustrated in FIG. 7. As also seen from this FIG. 8, the loss in each output CH increases with increase in the distance from the output CH located near the center, while with increase in the distance from the output CH located near the center, the shape of the loss peak becomes duller and the spectral width of the loss spectrum in each output CH also increases from W1 to W2 ( greater than W1) (distortion of wavelength characteristics); therefore, there was the problem that the wavelength separation accuracy considerably degraded when the signal wavelength band was totally shifted to the longer wavelength side or to the shorter wavelength side.
The present invention has been accomplished in order to solve the problem described above and an object of the invention is to provide an optical multiplexer/demultiplexer having structure for positively compensating for the distortion due to the aberration of wavelength characteristics or the like among the signal channels and permitting effective reduction of the loss variation or the like.
An optical multiplexer/demultiplexer according to the present invention is an AWG type optical multiplexer/demultiplexer applicable as a wavelength selection element to the WDM transmission systems, which comprises a substrate, at least one input waveguide provided on the substrate, a first slab waveguide, a plurality of channel waveguides, a second slab waveguide, and a plurality of output waveguides provided corresponding to respective signal channels.
In the optical multiplexer/demultiplexer according to the present invention, each of the above first and second slab waveguides has a predetermined slab length. The slab length is normally equal to a focal length of an optical input end functioning as a lens surface of each slab waveguide. The above input waveguide is a waveguide for guiding each of the signals of channel wavelengths set as signal channels at predetermined wavelength intervals, to the first slab waveguide, and an optical output end thereof is connected to the optical input end face of the first slab waveguide. The above channel waveguides are waveguides of mutually different lengths, which are flatly arrayed on the substrate in a state in which optical input ends thereof are connected to an optical output end face of the first slab waveguide so as to place the first slab waveguide between the channel waveguides and the input waveguide while optical output ends thereof are connected to the optical input end face of the second slab waveguide so as to place the second slab waveguide between the channel waveguides and the output waveguides. Further, the above output waveguides are waveguides flatly arrayed on the substrate in a state in which optical input ends thereof are connected to an optical output end face of the second slab waveguide, which are waveguides for individually taking out the signals of the channel wavelengths set at the predetermined wavelength intervals.
Particularly, the optical multiplexer/demultiplexer according to the present invention is characterized in that an optical input end of at least either one of the output waveguides is located at a position apart from the optical input end face of the second slab waveguide by a distance shorter than a focal length of the optical input end face of the second slab waveguide. It is preferable that the optical input ends of two output waveguides located outermost out of the output waveguides be placed on the circumference of a Rowland circle having a diameter equal to the focal length of the optical input end face of the second slab waveguide and that the optical input ends of the rest output waveguides excluding the two output waveguides out of the output waveguides be placed inside the Rowland circle and on a line connecting the optical input ends of the two output waveguides. In other words, it is preferable that the optical input ends of at least two output waveguides out of the output waveguides be located at positions where the optical output end face of the second slab waveguide intersects with the circumference of a Rowland circle having a diameter equal to the focal length of the optical input end face of the second slab waveguide. Here the line connecting the optical input ends of the two output waveguides located outermost out of the output waveguides may be part of a straight line (which corresponds to a chord of the above Rowland circle in this case) or part of a curve. Therefore, the optical output end face of the second slab waveguide may be a plane or a curved surface which is convex toward the optical input end face of the second slab waveguide. When the optical input ends of the two output waveguides located outermost out of the output waveguides are placed on the circumference of the Rowland circle in this way, it becomes feasible to decrease the loss variation among the channel wavelengths without increasing the total loss of the optical multiplexer/demultiplexer.
In the optical multiplexer/demultiplexer according to the present invention, it is preferable that extreme portions including the optical input ends of the respective output waveguides extend along a normal direction to the line connecting the optical input ends of the two output waveguides placed on the circumference of the Rowland circle.
When the optical input ends of the above output waveguides are placed on the chord of the Rowland circle, the optical input ends of these output waveguides are preferably arranged at unequal intervals in order to match the converging position of light of each channel wavelength with the center of the optical input end of each output waveguide. Specifically, it is preferable that the optical input ends of the output waveguides be placed so as to decrease the intervals between the optical input ends adjacent to each other from the center of the chord toward the both ends of the chord.
More specifically, the optical input ends of the output waveguides are placed on intersections between the chord and normals passing converging positions of respective light beams of the channel wavelengths converged at equal intervals on the circumference of the Rowland circle or on the circumference of a focal circle indicating converging positions of the light beams emerging from the optical output ends of the channel waveguides, out of normals to the chord of the Rowland circle. Namely, under such conditions that N is the number of the output waveguides, that r is a radius of the Rowland circle or a focal circle indicating converging positions of the respective light beams emerging from the optical output ends of the channel waveguides, and that xcfx86 is a central angle corresponding to an interval between the converging positions of the respective light beams of the channel wavelengths converging at the equal intervals on the circumference of the Rowland circle or the focal circle; the interval Ln between the optical input end of the nth output waveguide and the optical input end of the (n+1)th output waveguide out of the output waveguides flatly arrayed on the substrate is given by the following equations:
in the case of nxe2x89xa6N/2:             L      ⁢              xe2x80x83            ⁢      n        =          2      ⁢              r        ·        sin            ⁢                        ϕ          2                ·        cos            ⁢              {                              (                                          N                2                            -              n                        )                    ·          ϕ                }              ;
in the case other than nxe2x89xa6N/2:       L    ⁢          xe2x80x83        ⁢    n    =      2    ⁢          r      ·      sin        ⁢                  ϕ        2            ·      cos        ⁢                  {                              (                          n              -              1              -                              N                2                                      )                    ·          ϕ                }            .      
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.